This invention relates to improvements in antenna arrays, such as active slot antennas and the like, which can be configured to produce a set of directional far field radiation patterns suitable for use in determining the angular location of a target.
The use of radar to determine the distance to a target is well known. In a very simple system the time of flight of a signal transmitted from an antenna to a target and returning to the antenna provides an indication of the range of the target. This works well for targets that are very far away, but for close targets the time of flight may be too short to be analysed. In the case of automotive applications, for instance, the distance to a target is typically of the order of only a few hundred meters or less, making measurements based on time of flight impractical. In such applications alternative modulation schemes which monitor the phase of signals can be used. Examples of such schemes, which fall within the scope of the present invention, include frequency shift key modulation (FSK) radar, FMCW and LFMSK radar.
Steerable antennas which can produce directional far field radiation patterns to give directional sensitivity are also well known. FIG. 1 illustrates a typical non-directional beam pattern in which the far field strength at all angles is equal, illustrated in the form of a graphical representation of the amplitude (in decibels) of radiation from an antenna as a function of angle, and FIG. 1(b) shows a corresponding directional beam pattern sometimes referred to as a pencil beam. As can be seen the directional pattern is characterised by a power peak or main lobe 1 which extends in a defined direction and which subtends an angle θ to an axis of the antenna and is centred on an origin O. As shown the origin is located at the centre of the antenna. Within this application, the term beam pattern is used to describe both the shape of a far field pattern, encompassing the direction of the main lobe relative to an axis defined with respect to the antenna, as generated by an antenna in respect of transmit and or receive of radiation, and also the origin of the main lobe or power peak of the far field radiation along that axis.
A directional beam pattern such as the pencil beam of FIG. 1b can be achieved in many ways but one known antenna configuration, shown in FIG. 2, is called a phased array and employs an array of antenna elements Φ, each one connected to a source or detector 21 of radiation by a different phase offset, achieved by using different path lengths L from the source or detector 21 to each element Φ. In a simple phased array the elements each have a fixed phase delay and are permanently on. This provides a beam which has a fixed beam pattern. Generally the length of the array is defined in terms of the number of elements. The elements can take many forms.
In a refinement it is known to associate each element with an “on-off” switch, or other gain control device, which enables the element to be made active or inactive in response to drive signals applied to the switches by a processing unit. Each element may be independently switchable, and in this way the array can be modified to produce a variety of different beam patterns. Typically the processing unit will include a memory which stores a variety of switch values “On or Off” that have been derived empirically for a desired beam pattern, hereinafter called a radiation pattern.
There are many examples of such configurable antenna arrays known in the art. Many of these include elements which comprises areas of a conductive waveguide (such as a rod) through which radiation can escape by evanescent coupling. This coupling can be achieved in many ways, such as connecting regions of the waveguide to conducting patches formed on a drum or cylinder. Rotating the drum or cylinder varies the pattern of conducting patches on the rod, thus changing the regions through which radiation escapes. These regions controlled by patches fall within the term element as used in this application. Examples of such arrays are known from U.S. Pat. No. 6,750,827 and U.S. Pat. No. 5,959,589. Another example of such a radar apparatus is known from US 2007/0035433A1 to Waveband Corporation. As an alternative to the use of a rotating drum or cylinder, fixed conductive patches may be provided along the rod which are switched on or off using diodes or such like. Again, this falls within the scope of the term element as used in this application. A further example is known from U.S. Pat. No. 5,982,334 in which a semiconductor slab and plasma grating are used to cause evanescent coupling to occur in discrete regions along the length of a quartz rod, the regions defining antenna elements.
Looking again at FIG. 1b, a target 2 located along a line starting at the origin and lying in the direction θ will cause a strong echo signal to be received. If the target moves off that direction a weak echo is received. By producing a number of different beam patterns, each forming a different far field pattern, with the same origin but different directionality (e.g. a series of pencil beams which may overlap and extend collectively over a wide range of angles) and recording the amplitude of received echoes as the beams are applied one after the other, the beam pattern which produces the strongest echo can readily be identified, and hence the direction to the target can be determined from the directionality of that beam.
To provide a good resolution a large number of beams with highly directional far fields are needed, i.e. a large number of different beam patterns. To achieve this a large array, having a lot of antenna elements, is needed as a complex drive pattern involving many elements is needed. This makes the radar expensive and also bulky as there are constraints on the acceptable spacing between elements. Also, the more directional the beams (e.g. finer the pencil beams) that are used the more beams are needed to sweep across a given range of angles, the entire sweep being a radar “cycle”. Since each beam needs to be held in place for a given length of time in order to obtain high quality velocity information, a large number of beams may produce an unacceptably high radar cycle time.